1. Field of the Invention
The present invention relates to ultrasonic densitometer equipment using ultrasonic sound waves to measure bone integrity, and in particular, to an ultrasonic densitometer in which the ultrasound is conducted to a human heel through liquid filled bladders.
2. Description of the Prior Art
Various devices presently exist which may be used to measure the physical properties and integrity of a member such as a bone. Non-invasive density measuring devices can be used to determine cumulative internal damage caused by micro-crushing and micro-fracturing occurring in the bones of humans or animals such as race horses. Additionally, osteoporosis, or loss of bone mineralization, detection in humans and its cure or prevention are increasingly becoming areas of intense medical and biological interest. As the average age of the human population increases, a greater number of patients are developing complications due to rapid trabecular bone loss.
U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring the density of a bone structure, such as a finger bone or heel bone, to monitor the calcium content thereof. The device includes a pair of opposed spaced ultrasonic transducers which are held within a clamping device clamped on the bone being analyzed. A pulse generator is coupled to one of the transducers to generate an ultrasonic sound wave which is directed through the bone to the other transducer. An electric circuit couples the signals from the receive transducer back to the pulse generator for retriggering the pulse generator in response to those signals. The pulses therefore are produced at a frequency proportional to the transit time that the ultrasonic wave takes to travel through the bone structure which is directly proportional to the speed of the sound through the bone. The speed of sound through a bone has been found to be proportional to the density of the bone. Thus the frequency at which the pulse generator is retriggered is proportional to the density of the bone.
Another device and method for establishing, in vivo the strength of a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt, Jr. The device includes a launching transducer and a receiving transducer which are connected by a graduated vernier and which determine the speed of sound through the bone to determine its strength. The vernier is used to measure the total transit distance between the surfaces of the two transducers.
Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue, Tissue Characterization With Ultrasound, CRC publication 2, pp. 207-226) discusses various studies involving attenuation and speed of sound measurements in both cortical and spongy (cancellous or trabecular) bone. The results of these studies reveal a linear relationship between the wet sonic velocity and wet cortical density, and between the dry sonic velocity and the dry cortical density. The transit times of an acoustic signal through a bone member therefore are proportional to the bone density. Langton, et al. (Langton, C. M., Palmer, S. D., and Porter, S. W., (1984), The Measurement of Broad Band Ultrasonic Attenuation in Cancellous Bone, Eng. Med., 13, 89-91) published the results of a study of ultrasonic attenuation versus frequency in the os calcis (heel bone) that is utilized through transmission techniques. These authors suggested that attenuation differences observed in different subjects were due to changes in the mineral content of the os calcis. They also suggested that low frequency ultrasonic attenuation may be a parameter useful in the diagnosis of osteoporosis or as a predictor of possible fracture risk.
A common site for the ultrasonic measurement is the os calcis of the human heel. In one ultrasonic densitometer design for measuring this site, opposed ultrasonic transducers are placed on opposite sides of a receptacle sized to hold the human foot. The receptacle is filled with water of a controlled temperature which serves to couple the ultrasonic energy from a first transducer, through the gap between that transducer and the human heel, and from an exit point of the human heel back through a corresponding gap between the exit point and the receiving transducer. To the extent that the water approximates the acoustic impedance of the soft tissue surrounding the heel, coupling of ultrasonic energy through the heel is improved and the precision of the measurement increased.
While a water filled receptacle provides a simple mechanism for coupling ultrasonic energy to the heel, it may be desired to contain the water behind a flexible membrane or the like so as to reduce the possibility of spilling or contaminating the water. It is known to contain water within flexible bladders and to mount those bladders on an adjusting mechanism so that the bladders may be moved to compress the heel between them. This movement of the bladder supports may be augmented with limited inflation of the bladders and/or movement of the transducers. Such compressive type bladder systems create a risk of air entrapment between the bladders and the heel such as may affect the quality of the measurements. For this reason it is known to use a conical shaped membrane that initially contacts the heel at a single point and, with further compression, expands outward to reduce air entrapment. Such membranes are difficult to fabricate and do not conform well to essentially flat opposed surfaces of the heel.
The present invention provides a contained-water ultrasonic densitometer using stationary pre-inflated bladders. The bladders are arranged to form a cavity between them slightly smaller than the heel and the heel is slid in between the bladders. The sliding provides a wiping action that helps eliminate air trapped between the bladders and the foot.
Specifically, the densitometer provides pre-inflated bladders opposed along an ultrasonic propagation axis. A coupling material is contained within the bladders and ultrasonic transducers are positioned within the bladders to direct ultrasonic signals through the coupling material along an ultrasonic propagation axis between the transducers. The bladders are compliant so as to permit them to move apart to allow insertion of a human heel. The shape and composition of the bladder surfaces allow the bladders to slide over and conform with the heel while remaining substantially in alignment with the propagation axis.
Thus, it is one object of the invention to provide a contained-water ultrasonic densitometer having significantly decreased air entrapment between the bladders and the foot. The sliding action of the foot against the compliant membranes tends to reduce and eliminate entrapped air.
The first ultrasonic transducer may be held in opposition to a second ultrasonic transducer both with fixed separation and the first and second bladder surfaces may be mounted to a fixed support.
Thus it is another object of the invention to provide a greatly simplified mechanism for a contained water ultrasonic densitometer eliminating the need for a sliding mechanism to engage the bladders with the foot or repeated pumping and deflation of the bladders between measurements.
The bladder surfaces may be comprised of an elastomeric membrane having a surface coding of ultrasonic coupling gel.
Thus it is another object of the invention to provide a surface that easily slides along the surface of the human heel while also providing good coupling for the last interface between the bladder surface and the human heel.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.